Inside CostOS you have to add the users that you want to. So, in the top right corner of the screen, click on Usermanagement.
It is needed to add usernames that are the same with yours domain usernames. You don't need to add a password on our latest releases.
Then you could add as many users you like to.
Finally, when you try to login :
You just have to enable SSO login and then click the button login.